1. Technical Field
The disclosure relates to a flat speaker unit and a flat speaker device.
2. Related Art
Vision and audition are two most direct sensory responses of human beings. Thus, scientists have been dedicated to develop various renewable vision and audition related systems. Moving coil speaker is still the major product in the market among all the existing renewable speakers. However, along with people's increasing demand to high quality sensory enjoyment and the ever-decreasing sizes of 3C products (Computer, Communication, and Consumer Electronics), speakers having low power consumption, light weights, and small sizes that are designed according to human engineering, and such speaker can be used in either large-size flat speakers or small walkman headphones and stereo mobile phones, and in a foreseeable future, such technology may have a plenty of demands and application development.
The existing speakers can be categorized into direct and indirect types according to their radiation patterns or can be categorized into moving coil speaker, piezoelectric speaker, and electrostatic speaker according to the driving patterns thereof. The moving coil speaker is currently the most commonly used and most mature product. However, a moving coil speaker cannot be flattened due to a physical structure shortage thereof. Accordingly, moving coil speaker is not suitable for 3C products and home entertainment systems that have a developing trend of flattening.
A piezoelectric speaker pushes a membrane to produce sounds based on a piezoelectric effect of a piezoelectric material (i.e., the material is deformed when an electric field is supplied thereon). Such piezoelectric speaker has a flat and small structure. Main products of the electrostatic speaker in the market include hi-end earphones and loudspeakers. According to the operation principle of a conventional electrostatic speaker, a conductive membrane is clamped between two fixed porous electrode plates to form a capacitor, and by supplying a DC bias to the vibrating membrane and an AC voltage to the two fixed electrode plates, an electrostatic force generated by positive and negative electric fields drives the conductive membrane to vibrate, so as to produce sounds. The conventional electrostatic speaker requires a DC bias of up to hundreds or even thousands voltages, so that a high-price and large-size amplifier is required to be connected, which is a reason why the conventional electrostatic speaker is not popularized.
Audio is a major element in the future applications of flexible electronics. However, the flexible electronics has to have the characteristics of softness, thinness, low driving voltage, and high flexibility. Thus, how to break through the conventional design to fabricate elements having the characteristics required by the flexible electronics has become a major subject.